$1 Million Reward For Information That Leads To William Tyrrell

He's been missing for two years now.

It's been two years since William Tyrrell, aged three, disappeared from his grandmother's backyard and now the NSW Government and Police are offering a $1 million reward -- the largest in NSW's history.

Wearing a Spiderman costume, the boy was last seen in the garden in Kendall, on NSW's mid-north coast, on September 12, 2014.

NSW Police Commissioner Andrew Scipione said police believed he was kidnapped and hoped the reward would encourage people who knew something to come forward.

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William Tyrrell was wearing a Spiderman suit when he went missing.

"This investigation is not winding down, it's ramping up," Scipione said at a press conference on Monday.

"With a million dollar reward, there's a million reasons to come forward."

Despite the time that passed, Detective Chief Inspector Gary Jubelin said there was hope the boy would be found.

"Potentially we have a victim who could still be alive," Jubelin said.

Someone out there knows what happened to William and it's time they came forward so he can be returned to his mum, dad and sistePremier Mike Baird

He said the reward was only on offer for people who came to police.

"If we come to you, the offer of reward is off the table," Jubelin said.

Police were also wading through information relating to 600 new persons of interest which has been shared with local area commands across the state.

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Premier Mike Baird said it was time someone who knew something came forward.

"This is a little boy who has been tragically missing from his family for two years -- someone out there knows what happened to William and it's time they came forward so he can be returned to his mum, dad and sister," Baird said.

Several high-profile persons of interest have not led to arrests, including white goods repairman William "Bill" Spedding who denied any involvement in the child's disappearance and a woman photographed with a boy who looked eerily similar to William.